A conventional heat pump hot-water supply system is disclosed in Patent Document 1. The conventional system disclosed in Patent Document 1 includes a heat pump hot-water supply device that is operated with AC power supplied from a commercial power system, and an electricity storage device that converts an AC power of the commercial power system into a DC power to store the DC power and also convert the stored DC power into an AC power to supply the AC power. Accordingly, the AC power of the commercial power system is stored in the electricity storage device in the nighttime in which the electricity rate is lower than the daytime electricity rate, so that power required to operate the heat pump hot-water supply device during the daytime is supplied from the electricity storage device, thereby reducing the operating cost of the heat pump hot-water supply device.
However, a coefficient of performance (COP) of the heat pump hot-water supply device is associated with the outside air temperature, that is, the COP increases in proportion to an increase in the outside air temperature.
Meanwhile, a loss of the electricity storage device occurs due to power conversion upon storing (charging) and supplying (discharging) power in and from the electricity storage device. Taking into consideration the power loss due to the power conversion, when the outside air temperature is very high during the nighttime as in the summer season, the case where hot water obtained by operating the heat pump hot-water supply device during the nighttime is stored into a hot-water storage tank may reduce the operating cost (electricity price), compared to the case where the heat pump hot-water supply device is operated during the daytime using power stored in the electricity storage device.
Also, the power consumption of the heat pump hot-water supply device is comparatively much higher than that of electric devices such as TV sets or washing machines. Thus, in the case of a house provided with a solar light power generation system, when power generated by the solar light power generation system is preferentially consumed by the heat pump hot-water supply device, a deficient amount of power is supplied to the other electric devices. Hence, the amount of power supply from the commercial power system cannot but increase, undesirably increasing the electricity price.
Also, in the case of a house provided with a solar heat-assisted water heater, when the amount of hot water supplied from the solar heat-assisted water heater becomes deficient, the heat pump hot-water supply device should be operated using power supplied from the commercial power system, undesirably increasing the electricity price.    [Patent Document 1] Japanese Patent Application Publication No. 2005-164124